I'm afraid I don't get the Sidestep feat. In SRD, it allows you to take a 5' step whenever an opponent misses you. In effect, you're stepping aside from the blow as it misses you. In Codex, it says you can take a 5' step instead of making an attack of opportunity. That's almost the opposite of the SRD feat. Rather than moving when your opponent misses, you move by choosing not to make your own attack. That just doesn't seem like an actual sidestep to me, because you're not dodging anything. Am I missing something?

You are dodging their initial attack, since they spend a die to close (so that they may attack), and then you use your die to sidestep, thus requiring them to get close again by using another die. etc, etc.

I was referring to the Pathfinder SRD, sorry. I don't know if Sidestep is in the OGL SRD.

So are you guys referring to when someone tries to enter grapple range? I didn't think of that when reading the feat, and it doesn't mention that. I was thinking about, for example, when your opponent moves out of a threatened square and provokes an AoO from you. There is no attack, and therefore nothing to sidestep.

Edit: I just noticed that the one-liner description below the feat name says you use it when your opponent attacks. That explains it. It could probably be clearer, though.